As shown in FIG. 13, a hip joint in which a pelvis 61 is connected with a femur 62 by a caput 63 which works as a spherical bearing, enables the movements such as uprightly standing, walking, sitting-down and the like. The femur 62 has a greater trochanter and lesser trochanter at the proximal portion thereof, and gluteus medius 65 connected with the greater trochanter 64 at the epiphysis controls movements of the femur. The break of spherical bearing due to an accident or the like loses the cooperation of the femur 62 and the pelvis 61 so that the leg comes to be uncontrollable and is not able to support the ordinary daily life.
The reproduction of the spherical bearing and the introduction of the supporting mechanism thereof are required in order to recover the function of the hip joint. The spherical bearing, not illustrated in the figure, consists of a socket fixed to the pelvis and a spherical head which cooperates with the socket so as to operate as an articulation, the supporting mechanism is a stem which supports the spherical head to transmit the load to the femur. The socket and spherical head are required high wearability and durability enough to be used as a bearing. Recently, the development of the material thereof has been remarkably advanced as well as the performance thereof has been greatly improved, so it seems that the technology of the spherical bearing has just been grown up.
The stem is a short rod 66, as shown in FIG. 13 (b), which is implanted into the femur 62, being inserted into a deep hollow 67 extending from the epiphysis to the diaphysis so as not to pass through the greater trochanter 64. As shown in FIG. 13 (c), the deep hollow is formed by excising some spongiosa 69 and a part of medulla in the cortical bone 68 having high stiffness by means of a medical rasp, so as to have the depth reaching to medullary cavity 70. The stem 66 consists of a neck 66a for introducing a load from the spherical head 71, of a body part 66b for supporting the neck and transmitting the load to the femur, and of a leg 66c for guiding the insertion of the stem into the hollow and keeping the attitude thereof while advancing, as shown in FIG. 13 (d).
A stem ought to be chemically stable, harmless in human bodies, durable, and transmit a load to a femur effectively. It is often made of cobalt alloy and titanium ally and is generally formed axial unsymmetrical, whose external surface is often close to the cortical bone in order to ensure the load transmission, for the body part 66b has to be provided with a shoulder 66d in the epiphysis 72 where there is an unremovable greater trochanter 64, as shown in FIG. 13 (c). FIG. 14 (a) is the graphic arts of the stem 66, and (b) shows a femoral prosthesis while the spherical head 71 is engaged with the stem, however, such a complex shaped stem made of the hard metal mentioned above has made the manufacturing cost very expensive.
It is impossible to excavate a deep hollow that agrees perfectly with a ready-made stem in the femur which is different in every patient. Therefore, a method for uniting the stem with the femur by the cement filled in an oversized deep hollow formed in the femur is adopted, or a method where the stem is tightly inserted into an undersized hollow to unite with the femur by Bone-Growth is adopted. Of course, the shape and the stiffness of the femur are considered for every patient in both methods. The former requires less time for uniting them than the latter does, but un-reacted monomer may lixiviate from the cement, and taking off the stem from the femur is very hard during the re-operation, furthermore, there is a high possibility of suffering from pulmonary embolism, so that recently the latter method has also been researched well to develop the artificial hip prosthesis stem.
The load acted on the hip joint is transmitted as a shearing stress occurring on the interface between the stem and the femur. The shearing stress in the longitudinal direction and the rotational direction has to be evaluated with high accuracy and both the shearing stress and the distribution thereof on the interface have to be corresponding to transmitted load required. Even if the integrated value of the stress acted on the interface agrees with the transmitted load, the stress concentration occurring locally on the interface causes damage of bone and/or sharp pain to patients in the case that the stress is larger than the allowable strength of the surface corresponding thereto. The phenomenon described below will be helpful to understand it.
FIG. 15 (a) shows a modeling of the stem and the femur, wherein the stem is regarded as an inner cylinder 66A and the femur as an outer cylinder 62A. The modelings of (b) and (c) of FIG. 15 show the state where the stem is inserted tightly into the deep hollow of the femur. Although the interface 73 is depicted to be thicker than the actual interface, following phenomena will occur in an actual thin interface. (b) is an example where either compression or tension acts in the vertical direction. When both of the outer cylinder 62A and inner cylinder 66A are isotropic, the shearing stress is concentrated on the both ends 73e of the interface 73, as shown by the distribution of many straight lines of which length is corresponding to the magnitude of stress. The straight lines are drawn by the radial straight lines so as to be clear to see though they should be drawn in the axial direction.
FIG. 15(c) is the example where the torsion is acted on the both cylinders and the torsional shearing stress is concentrated on the both ends as well. Both cases of (b) and (c) teach us that no stress concentration, little fluctuation of stress and extremely low stress occur on the intermediate portion. If the end of interface of the deep hollow cannot withstand the load under such a distribution, it means that the stem cannot be applied to a patient in spite of the fact that the overload is not charged on him. The fact that the shearing stress occurring on the interface is distributed like this has already disclosed in a book authored by R. J. Schliekelmann, a Dutch aircraft engineer.
While a human uprightly standing or walking, not only a load in the vertical direction is acted on the hip joint but also bending stress is acted thereon since the caput 63 deviates from the longitudinal axis of the femur 62, as shown in FIG. 13 (a). The bending moment causes the tension on one side of an object and the compression on the other side thereof, so that both the load in the vertical direction and the bending stress fundamentally makes a stress mechanism caused by the shearing stress acted on the interface as shown in FIG. 15(b). The motion during sitting-down makes a stress mechanism caused by the torsional stress as shown in FIG. 15(c).
The thicker the interface becomes, the duller the phenomena of FIG. 15 become. Nevertheless, if a stem made of isotropic material, e.g., titanium alloy, is implanted into a femur, both the compressing and torsional shearing stresses greatly occur at the end of the proximal portion which is close to the heart and at the end of the distal portion which is away from the heart. Most of the load transmitted to the femur is concentrated to the upper end (the end of proximal portion) of the main body and the end of the thin leg (distal portion).
Thereby, the load to be charged on the hip joint is transmitted also to the distal portion, so that the route for transmitting the load comes to be different from that of the load in a normal state. The reduction of the load charged on the proximal portion which is activated under the appropriate load causes Stress-Shielding that makes the cortical bone thinner and the density thereof lower. Besides, if the stem loosens at the proximal portion, Micro-Movement occurs to make the metallic worn powder therefrom, which may be carcinogenic. Since it is not easy that the figure of metallic stem agrees with that of the deep hollow, Fit and Fill, which mean a fitting rate to the wall of a deep hollow and a filling rate in the cross section of a hollow, are not expected to be very high. These phenomena force the femur to be charged by spotty loads on the contacting surface independently of the distribution of load in FIG. 15, causing the destruction of bones and interfacial separation. Moreover, the serious problem on using the metallic stem is the metallic fatigue caused by applying it to the part of movable structure.
In WO2005/034818A1, a composite material stem is disclosed so as to solve the problem of the metallic stem mentioned above. The principal reason why much attention is drawn to the composite material stem is that fiber reinforced plastics are free from fatigue phenomenon. The cited invention is different from the stem having the metallic core covered with fiber reinforced plastics as disclosed in WO93/19699A1 and U.S. Pat. No. 6,749,639 B2. e.g., the whole of stem is made of composite material and the rigidity of the skin thereof is varied in the longitudinal direction, moreover the structure of the stem is epoch-making, wherein even the core of the stem contributes to the control of epidermis of the stem, which is fundamentally different from U.S. Pat. No. 4,892,552 wherein the stem is cut out from the overlaid block although it is made of composite material only. FIG. 16 is a cross sectional view of the complete composite material stem 74 according to the cited invention, wherein it is implanted to the femur 62, having a simple outer shape without shoulder 66d of the metallic stem 66 which is additionally shown by single-dotted chain lines in FIG. 16(a).
FIG. 16 (a) is a cross sectional view of the stem in the femur observed at the front of a human body, and (b) is at the side of the body. A cortical bone 75 is depicted by double-dotted chain lines around the stem 74, but spongiosa is not. It is worthy of notice that the epidermis 77a of the stem is very thin in the diaphysis 76 and the epidermis 77b is thick in the epiphysis 78, so that the rigidity of the stem from the proximal portion toward the distal portion is gradually decreased by changing the thickness of layer of the composite material between the epidermis 77 and the inner epidermis 80 which give a cavity 79 to the core portion of the stem.
The construction which gives such a change of rigidity is realized by changing the number and the direction of layers of reinforced fiber which forms the epidermis. The foaming resin 81 occupies the space surrounded with the inner epidermis 80 and the epidermis 77a, promoting the shape of the whole stem stable. Since the interfacial stress does not occur locally or greatly at the part where the load can not be transmitted due to the low rigidity thereof, the stress concentration at the distal end is avoidable.
Therefore, the stress concentrates inevitably to the proximal portion, so that the route for transmitting the load can be made to be close to that in a normal state. Since the metallic stem mentioned above is isotropic, the rigidity thereof cannot be decreased at both the proximal end and the distal end to the same degree of that of cortical bone, resulting in that the high shearing stress caused by the tension/compression and the torsion can not be checked at both the ends of the stem. Compared with the metallic stem, the composite material stem enables us easily to have measures to check the concentration of the shearing stress.
However, in the case of the composite material stem overlaid with fiber woven cloth and/or uni-directional fiber for reinforcement and the resin as the matrix, the shoulder which the metallic stem always has forces the fiber woven cloth wrinkle so that the laminating work comes to be rather difficult. Therefore, in order not to decrease the molding quality and not to increase the quality unstable, the shape of the stem should be simple without such a sharp curve as the metallic stem shown by the reference number 66 in FIG. 16 (a) has. However, Fit and Fill at the epiphysis will decrease if the stem does not have a shoulder, resulting in increase in the contacting area with the spongiosa having low durability and in requirement of much time and technical skill for inserting the stem into the deep hollow being curved complicatedly.
Also, referring to FIG. 16 (a), in the epiphysis 78, the connecting force at the side of the greater trochanter is low, but epidermis 77B having a high rigidity at the counter side of the greater trochanter i.e., at the medial side, makes the stress concentrate at the proximal end according to the principle of FIG. 15. The break of caput often makes the cortical bone 75B at the medial side weak and the cutting off a part of the cortical bone during the operation makes the strength thereof low, moreover, the cortical bone is charged with the heaviest load when the bending moment acts on the stem by the load charged on the spherical head after the operation. Therefore, the cortical bone 75B is damaged when the stress concentrates on the weakened part, so that it will lose the property to withstand the load, resulting in the failure of the function as an artificial hip joint.
It is very difficult to decrease in the rigidity of epidermis 77 at the epiphysis of the composite material stem, for the epidermis is extended to the neck 66a which supports the spherical head 72 and the cavity 79 cannot be extended till the vicinity of the neck where the rigidity has to be high, consequently, the artificial hip joint of FIG. 16 can not be always applied to the patients of osteoporosis or the like, resulting in the limit of application thereof.
In order to fit a stem to the hollow of a patient femur the shape of the hollow has to be determined according to the data of three-dimensional CT graphics of a femur. The outer shape of epidermis of the stem touching the wall of the hollow has to be formed with high accuracy. The process to form the stem cannot be easily divided, for it consists of single parts, or many complicated processes are required. Not only the pressure control in the mold is difficult but also the miniaturization and lightening of the mold does not progress. Even partially, ready-made stems are not still usable consequently, inexpensive stems are unavailable.